Arrays in Java

by Ben Davidson

Objectives

• Understand the importance and practical uses of arrays
• Learn how to declare, create, and initialize an array
• Understand how an index in an array works
• Use a for loop with an array
• Understand multi-dimenstional arrays
• Become familiar with common array library functions

Prerequisites

• Understand Java primitive types
• Understand execution flow and control statements
  • Sequence
  • Selection (if statement)
  • Repetition (while, for, do while statements)

When to Use an Array

• Arrays are an important and useful tool in most programming languages.
• An array can be loosely defined as a collection of variables.
• Each variable in an array is called an element.
• Instead of working with many variables of the same type, you can work with an array which holds many variables (elements).
• Some cases in which an array would be useful are:
  • You are to write a program that averages the test grade of 100 students. Instead of declaring and using 100 variables, you can store 100 values in an array.
  • You need to keep track of your employees' names. Instead of declaring many String variables, you can store them in a single array of Strings.

Declaring an Array

• Before using any variable, including an array, in a Java program, you must declare that variable.
• Syntax:
  dataType[] arrayName;
• Examples:

  int[] testScores;
  double[] dailyTemps;
  float[] accountBalances;

Creating an Array

• Declaring an array in Java does not cause the computer to reserve space for the elements of that array.
• You must use the new keyword to create the array.
• Syntax:
  arrayName = new dataType[arraySize];
• Examples:

  double[] empSalaries;         // array declaration
  empSalaries = new double[6];  // array creation
  
  int dataLength = 15;
  int[] data;                  // array declaration
  data = new int[dataLength];  // array creation

Initializing the Values in an Array

• It is possible to both declare and create an array on a single line.
• Syntax:
  dataType[] arrayName = new dataType[arraySize];
• Examples:

  double[] empSalaries = new double[6];
  
  int dataLength = 15;
  int[] data = new int[dataLength];

• Each element in an array created as shown above will automatically be initialized to 0, null, or false depending on the data type.
• It is possible to declare, create, and initialize the values in an array on one line.
• Syntax:
  dataType[] arrayName = { value0, value1, value2, . . . };
• Examples:

  String[] employees = { "Bob", "Larry", "Ben", "Louise" };
  double[] sizes = { 13.8, 22.1, 18.6, 18.5, 17.4 };

Array Length

• All Java arrays have an attribute named length which is the number of elements contained in the array.
• Syntax:
  arrayName.length
• Example:

  String[] array = { "Ben", "Emily", "Albert", "Lydia" };
  int len = array.length;  // len will hold 4

• Arrays in Java have a fixed length, or in other words the length of an array in a Java program cannot change while that program is running.

Array Indices

• Every variable in an array, called an element, has an index or location.
• The first index of a Java array is always 0.
• The last index of a Java array is always the length of the array minus 1.
• You can store or retrieve individual values in an array by using the index.
• Examples:
  

  double[] sizes = { 13.8, 22.1, 18.6, 18.5, 17.4 };
      // 13.8 is stored at index 0 (the first index).
      // 17.4 is stored at index 4 (the last index).
  
  sizes[2] = 15.9;  // changes the value at index 2 from 18.6 to 15.9
  
  double result = sizes[3] + sizes[1];  // adds the values at indices 3 and 1.
  

• Trying to access an element before the first or beyond the last index of an array causes an ArrayIndexOutOfBoundsException.
  

  double[] sizes = { 13.8, 22.1, 18.6, 18.5, 17.4 };
  sizes[-1] = 14.1;  // Causes an ArrayIndexOutOfBoundsException
  sizes[5] = 15.0;  // Causes an ArrayIndexOutOfBoundsException

Arrays and the For Loop

• When working with arrays, you will often use a for loop.
• Examples:

  // Printing the contents of an array:
  String[] empNames = { "Heath", "Garth", "Chandra", "Susan" };
  for (int i = 0;  i < empNames.length;  i++) {
      System.out.println(empNames[i]);
  }
  
  // Finding the largest element in an array:
  double[] hourlyWage = { 7.75, 8.00, 9.00, 8.50 };
  double max = hourlyWage[0];
  for (int j = 1;  j < hourlyWage.length;  j++) {
      if (hourlyWage[j] > max) {
          max = hourlyWage[j];
      }
  }

Multidimensional Arrays

• Arrays can hold any data type, including other arrays.
• A multidimensional array is simply an array of arrays.
• Example:
  char[][] ticTacToe = { {'x', 'o', 'x'}, {'o', 'x', 'o'}, {'o', 'o', 'x'} };
• An array can have as many dimensions as you want.

Library Functions

• The standard Java class java.util.Arrays contains many useful methods for manipulating arrays, including: asList, binarySearch, copyOf, equals, fill, sort, and toString.
• To learn more about each method read the online documentation (called java docs) at http://download.oracle.com/javase/6/docs/api/java/util/Arrays.html
• Examples:

  Code		Output
  import java.io.PrintStream; import java.util.Arrays; public class TestLibFuncs {     public static void main(String[] args) {         PrintStream out = System.out; // Use shorter syntax.         String[] vegies = { "radish", "carrot", "tomato", "pea" };         out.println("vegies " + Arrays.toString(vegies));         String[] copy = Arrays.copyOf(vegies, vegies.length);         out.println("copy " + Arrays.toString(copy));         out.println("equal? " + Arrays.equals(vegies, copy));         vegies[2] = "pepper";         out.println("vegies " + Arrays.toString(vegies));         out.println("copy " + Arrays.toString(copy));         out.println("equal? " + Arrays.equals(vegies, copy));         Arrays.fill(vegies, "potato");         out.println("vegies " + Arrays.toString(vegies));         vegies = Arrays.copyOf(copy, copy.length);         Arrays.sort(vegies);         out.println("vegies " + Arrays.toString(vegies));         out.println("location of pea? " + Arrays.binarySearch(vegies, "pea"));         out.println("location of okra? " + Arrays.binarySearch(vegies, "okra"));     } }		vegies [radish, carrot, tomato, pea] copy [radish, carrot, tomato, pea] equal? true vegies [radish, carrot, pepper, pea] copy [radish, carrot, tomato, pea] equal? false vegies [potato, potato, potato, potato] vegies [carrot, pea, radish, tomato] location of pea? 1 location of okra? -2